I used the Viva Solista Mk1 for 5 years, a very nice, if very large, heavy, inefficient, hot running amp. I liked it a lot, but also understand the Mk 2 is significantly better and more expensive.
Phaelon is correct about the tubes, the Solisata runs at a very high plate voltage, which burns out many 845 tubes rapidly.
I know you dod'nt qask for the comparison, but I know use a simiar output Ayon Spark. It's cheaper and I prefer it.
What reviews I have read of Mastersound amps, were OK, but not glowing, if you'll excuse the pun.